Frame Dropping vs AV-Sync?
As far as I can see:


No Sync just tries to play back every single Frame no matter if it causes lags or time shifting problems with the Audio or the actual physical time.
Frame Dropping drops Frames from the ViewPort Animation Playback to match up with the actual Frame Rate being used
AV Sync skips Frames from Video Footage in the VSE so that Audio and Video in the Image Preview Sequencer Mode stays in sync